1. Field of the Invention
This invention relates to a field emission electron source for emitting electrons due to the field emission using a semiconductor material without heating and a method of manufacturing the same. More particularly, the present invention relates to a field emission electron source applicable to a planar light emitting apparatus, a display apparatus, and a solid vacuum device, and a method of producing the same.
2. Prior art
As field emission electron sources, those using the so-called Spindt type electrode such as disclosed in, for example, U.S. Pat. No. 3,665,241 are well known. The Spindt type electrode comprises a substrate having a multitude minute emitter chips of a triangular pyramid shape disposed thereon and gate layers that have emission holes through which tips of the emitter chips are exposed and are insulated from the emitter chips. In this structure, a high voltage is applied in a vacuum atmosphere to the emitter chips as negative electrode with respect to the gate layer, electron beams can be emitted from the tips of the emitter chips through the emission holes.
However, the production process of the Spindt type electrode is complicated and it is difficult to make a multitude of emitter chips of a triangular pyramid shape with high accuracy and hence, difficult to make a device of large emission area when applying this technology to, for example, a planar light emitting apparatus or a display apparatus. Also with the Spindt type electrode, since the electric field is concentrated on the tip of the emitter chip, emitted electrons ionize various residual gases into positive ions where the degree of vacuum is low and the residual gas exists in the vicinity of the tips of the emitter chips. Therefore, the positive ions impinge on the tip of the emitter chips and eventually damage the tips of the emitter chips, resulting in such problems that the current density and efficiency of the emitted electrons become unstable and the service life of the emitter chips decreases. Thus, the Spindt type electrode has such a drawback that the atmosphere in which it is used must be pumped to a high degree of vacuum (10xe2x88x925 Pa to 10xe2x88x926 Pa) in order to avoid the problems described above, resulting in higher cost and difficult handling.
In order to overcome the drawback described above, field emission electron sources of MIM (Metal Insulator Metal) type and MOS (Metal Oxide Semiconductor) type have been proposed. The former is a field emission electron source of a planar configuration having a laminated structure of metal-insulation film-metal and the latter is the same structure one of a metal-oxide film-semiconductor. However, it is necessary to reduce the thickness of the insulation film or the oxide film in order to improve the electron emitting efficiency to thereby increase the number of electrons emitted with these types of field emission electron sources, while making the insulation film or the oxide film too thin may lead to dielectric breakdown when a voltage is applied between the upper and lower electrodes of the laminated structure described above. Thus there has been such a problem that, in order to avoid the dielectric breakdown of the insulator film, the electron emitting efficiency (pullout efficiency) cannot be made too high because there is a limitation on the reduction of the thickness of the insulation film or the oxide film.
A different field emission electron source has recently been proposed in Japanese Patent Kokai Publication No. 8-250766. According to this publication, the field emission electron source is made by using a single-crystal semiconductor substrate such as a silicon substrate, forming a porous semiconductor layer (a porous silicon layer, for example) by anodization of one surface of the semiconductor substrate, and forming a surface electrode made of a thin metal film on the porous semiconductor layer. A voltage is adapted between the semiconductor substrate and the surface electrode to cause the field emission electron source (semiconductor cold electron emitting device) to emit electrons.
However, in the structure disclosed in Japanese Patent Kokai Publication No. 8-250766, there is such a drawback that the popping phenomenon is likely to occur during electron emission. In the field emission electron source in which the popping phenomenon is likely to occur during electron emission, the unevenness in amount of electrons emitted is likely to occur. Thus, when this type of field emission electron source is used in a planar light emitting device and a display apparatus, there is such a drawback that the light is not emitted uniformly.
Then, the inventors studied whole-heartedly the above drawbacks and found out that in the field emission electron source as disclosed in Japanese Patent Kokai Publication No. 8-250766, since a porous silicon layer formed by making the entire surface of the single crystal substrate on the principal surface side porous constructs a strong electric field drift layer into which electrons are injected, the strong electric field drift layer has a heat conductivity lower than that of the crystal substrate and the field emission electron source has a high thermal insulating characteristics, which results in that the temperature of the substrate rises relatively largely when voltage is applied and current is flown. Further the inventors found out that electrons are thermally excited and electrical resistivity of the single-crystal semiconductor substrate decrease when the temperature of the substrate increases, accompanied by increase of the amount of electrons emitted. Therefore, this structure is susceptible to the popping phenomenon during electron emission leading to unevenness in amount of electrons emitted.
Based on the above findings, the present invention has been accomplished. That is, the object of the present invention is to provide a field emission electron source capable of achieving a stable emission of electrons with high efficiency at a low cost and a method of producing the same.
In order to achieve the above-mentioned object, according to a first aspect of the present invention, there is provided a field electron source comprising an electrically conductive substrate having principal surfaces; a strong electric field drift layer formed on one of the principal surfaces of said electrically conductive substrate and a surface electrode made of a thin electrically conductive film which is formed on said strong electric field drift layer, wherein a voltage is applied to said surface electrode used as a positive electrode with respect to said electrically conductive substrate, thereby electrons injected from said electrically conductive substrate being drifted in said strong electric effect drift layer and emitted through said surface electrode, characterized in that said strong electric field drift layer comprises at least a) semiconductor crystal regions formed in a manner to stand up vertically on said principal surface of the electrically conductive substrate and b) semiconductor micro-crystal regions having nano-structures intervened between the semiconductor crystal regions coated with an insulating film which has a thickness smaller than the crystal grain size of said semiconductor micro-crystal region and is formed on the surface of the semiconductor micro-crystal. Therefore, 1) the dependency on the degree of vacuum of the electron emission characteristic is low and no popping phenomenon occurs during the electron emission. Also the electrons can be emitted with a high stability and a high efficiency. 2) As the electrically conductive substrate, the semiconductor substrate such as a single-crystal silicon substrate and the substrate such as a glass substrate with a conductive film formed thereon can be used, in which case it is made possible to achieve larger emission area and lower production cost than in the case of using the conventional porous semiconductor layer and of the Spindt-type electrode, as in the conventional example.
In the present invention, said semiconductor crystal is preferably polysilicon. But other single crystal, poly-crystal and amorphous semiconductor, for example, poly-crystal semiconductor of IV group, IVxe2x80x94IV group compound semiconductor such as SiC, III-V group compound semiconductor such as GaAs, GaN and InP, and II-VI group semiconductor such as ZnSe may be used.
In the present invention, the semiconductor micro-crystal region is formed by making the single crystal or poly-crystal semiconductor porous by the anodization, which constructs a drift region; the details thereof are described in U.S. patent application Ser. No. 09/140,647, now U.S. Pat. No. 6,249,080, the content whereof is incorporated in this specification by reference. The insulating film preferably made of an oxide film or a nitride film.
In order to achieve the above-mentioned object, according to a second aspect of the present invention, there is provided a field electron source comprising an electrically conductive substrate, a strong electric field drift part formed on one of the principal surface of said electrically conductive substrate and a surface electrode of a thin metal film formed on said strong electric field drift part, wherein a DC voltage is applied to said surface electrode used as a positive electrode with respect to the electrically conductive substrate, thereby electrons injected from the electrically conductive substrate being drifted in said strong electric effect drift part and emitted through said surface electrode, wherein the strong electric field drift part preferably comprises drift regions in which the electrons are drifted and heat radiation regions which have a heat conductivity higher than that of the drift region, the drift regions and the heat radiation regions are mixed and distributed uniformly. In a typical case, the drift regions has a mesh-like cross section at right angles to the direction of thickness of the electrically conductive substrate and, the heat radiation regions which are built up in the mesh openings. Therefore, the heat generated in the drift region is radiated through the heat radiation region in the strong electric field drift part and thus, no popping phenomenon occurs during the electron emission and the electrons can be emitted with a high stability and a high efficiency.
The drift region may be a layer made by alternately laminating layers whose porosity are different from each other in the direction of thickness of the electrically conductive substrate, thereby the efficiency of the electron emission can be enhanced. And said drift region may be a layer whose porosity changes continuously in a direction of thickness of the electrically conductive substrate, thereby the efficiency of the electron emission can be enhanced.
The openings or void of the mesh-like drift region is preferably in the shape of a minute polygon or a minute circle.
The drift regions and heat radiation regions may be selected from the group consisting of a single crystal, poly-crystal and amorphous of silicon or silicon carbide. The heat radiation region is preferably a silicon or silicon carbide with an insulating film on the surface thereof and therefore, the heat radiation region has a high heat conduction characteristic and a electrical insulating characteristic, resulting in the increase of heat radiation. The insulating film is preferably an oxide film or a nitride film.
The surface electrode is preferably made of a thin metal, but transparent and conductive films of ITO, SnO2 and ZnO2 can be used for the surface electrode.
The electrically conductive substrate is preferably a substrate on one of the principal surface of which the electrically conductive film is formed and therefore, it is made possible to achieve larger emission area and lower production cost than in the case of using a semiconductor substrate such as a single-crystal silicon substrate as an electrically conductive substrate.
In order to produce the field emission electron source, a part of the semiconductor region on the principal surface of the electrically conductive substrate is made porous by anodization in the direction of thickness, and then the semiconductor region and the porous semiconductor region are oxidized to form a heat radiation region and a drift region, finally a surface electrode made of a thin metal film being formed on the strong electric field drift part comprising the drift region and the heat radiation region.
Because a part of the semiconductor region on the principal surface of the electrically conductive substrate is made porous and then oxidized, the drift region and the heat radiation region can be formed using the same semiconductor material. Therefore, it is not necessary to form the drift region and the radiation region separately from the beginning of the preparation and the shape of the pattern of the drift region and the heat radiation region can be easily controlled. As a result, a field emission electron source in which no popping phenomenon occurs during the electron emission and electrons can be emitted with a high stability and a high efficiency can be achieved at a low cost.
Where said anodization is effected, 1) if the anodization is carried out after a column-like poly-crystal semiconductor layer stood up vertically on the surface of the electrically conductive substrate was made, mixture structure of the semiconductor crystal regions and the semiconductor micro-crystal regions can be easily made. 2) If the mask has cross-section shape of a mesh with openings like a minute polygon is arranged on an area on which the heat radiation region is to be formed on the semiconductor region and then, the anodization is effected, with the result that only the part of the semiconductor region on the surface of the electrically conductive substrate which corresponds to the drift region can be made porous by anodization. Also, in the case of that the mask has a cross section of a mesh with openings in the shape of a minute circle is arranged on the area on which the heat radiation region is to be formed on the semiconductor region and then, the anodization is effected, only the part of the semiconductor region on the surface of the electrically conductive substrate which corresponds to the drift region can be made porous by anodization.
Where said anodization is effected, 3) the magnetic field is applied to the electrically conductive substrate during the anodization in such a manner that the rate making the semiconductor region porous in the vertical direction to the one surface of the electrically conductive substrate is much faster than that in the other directions, with the result that the anisotropy in the rate of making the semiconductor region porous is enhanced. That is, in the region which is to be a drift region by oxidation after making porous, the anisotropy in the forming rate of the porous layer during the anodization is enhanced. Therefore, the controllability in the shape in the horizontal direction and in the direction of thickness of the drift region can be enhanced, with the result that the minute patterns of the-drift region and the heat radiation region can be formed with a good controllability in the direction of thickness.